The
Devonian ExtinctionToward
the end of the Devonian, nearly 70% of all invertebrate species vanished
during the Late Devonian extinction. Marine species
(especially tropical ones) suffered the most extinctions, followed
by freshwater species, whereas terrestrial species were hardly affected.
In particular, tabulate coral/stromatoporoid reefs vanished entirely,
leading to a great decline in worldwide coral reef-building until
the Triassic Period. Many species of brachiopods, trilobites, and
early fishes went extinct, as did the planktonic graptolites.
These extinctions were not the result of a single major extinction
event, but rather smaller extinction events that occurred over a period
of more than 20 million years.

The causes of the extinctions are uncertain, although there are several
viable hypotheses. Most of these focus on the theory that the expansion
of terrestrial plants decreased the amount of carbon dioxide in
the atmosphere through two separate processes. One process is chemical
weathering, a process that uses up carbon dioxide from the atmosphere, and that
plants accelerate. The second process is carbon storage. Plants fix
carbon dioxide from the atmosphere and convert it to organic carbon,
stored in their stems, roots, and leaves. After plants die the organic
carbon can be released to the soils. Both of these processes would
have increased as terrestrial vegetation spread, reducing the amount
of carbon dioxide in the atmosphere, triggering global cooling and
eventually glaciation. An asteroid impact may also have occurred at
this time.